One of the key challenges in scaling MOSFET devices down to 25 nm is to improve the drive current without degradation of the short channel performance and off-gate leakage current. Specifically, in sub-100 nm MOSFET devices, the deleterious impact of short channel and hot carrier effects becomes more severe, due to reduction in gate length. Therefore, doping in the channel region (e.g., halo doping) must be increased in order to suppress the short channel and hot carrier effects. The increased channel doping nevertheless leads to degradation in carrier mobility, which in turn reduces the drive current in the channel region. Improvement in the drive current is therefore limited.
U.S. Pat. No. 5,777,364 issued on Jul. 7, 1998 entitled “Graded Channel Field Effect Transistor” describes a metal-insulator-semiconductor field effect transistor (MISFET) with a SiGe channel region having a vertically germanium content gradient. An example of the MISFET, as disclosed by this patent, is shown in FIG. 1 (see reference numeral 110), which is formed in an upper surface of a silicon substrate 112 and contains a narrow in situ doped silicon layer 114, a narrow undoped silicon spacer 116, an undoped SiGe channel layer 118, a silicon cap layer 120, a gate insulator layer 122, and a gate electrode layer 124. The source and drain regions 126 and 128 of the MISFET 110 are formed by implantation or out-diffusion, defining a channel region 127 therebetween. Contacts 130, 132 and 134 are formed on the source, drain, and gate regions to complete the device. The percentage of germanium in the SiGe channel layer 118 is graded along the vertical direction, i.e., the direction perpendicular to the substrate surface, to form a single peak percentage level. Electric current flows between the source and drain regions 126 and 128 along a lateral direction, i.e., the direction parallel to the substrate surface, through the channel region 127. The vertical gradient of the germanium content in the SiGe channel layer 118 therefore provides a built-in electric field along the vertical direction and functions to confine the carriers to a desired vertical location in the channel layer, without little or no parasitic flow of carriers at the insulator/channel interface or at the spacer/channel interface. In this manner, the transconductance of the MISFET device 110 is improved in comparison with devices having only un-graded SiGe channels.
However, the MISFET device disclosed by U.S. Pat. No. 5,777,364 only provides means for passively confining the carriers to a desired vertical location in the channel region, which does not in any manner improve the effective mobility of the carriers along the lateral source/drain direction or the current flow direction.
There is therefore a need for increasing the effective carrier mobility in the channel regions of the sub-100 nm MOSFET devices along the source/drain direction or the current flow direction, without otherwise degrading the performance of the MOSFET devices.